1. Field of the Invention
The present invention relates to multilayer plastic articles displaying certain desirable qualities, and more particularly to methods and apparatus for producing such articles.
2. Description of the Prior Art
Injection molding is a preferred fabrication method for a variety of plastic articles, and possesses a number of well known advantages over common alternative methods of fabrication. Chief among these advantages is the reduced amount of scrap which must be disposed of or recycled, and the relative speed with which plastic articles can be made compared to the aforementioned alternative methods of fabrication. Injection molding has been found particularly suitable for making precision parts of complex geometry at relatively high production rates.
It has been appreciated that articles formed by injection molding or by other forming processes and comprising overlapping layers of diverse thermoplastic resinous materials could realize a combination of properties which could not be realized otherwise, except perhaps by the synthesis of an entirely new material or by the use of a prohibitively expensive existing material.
Heretofore, it has not been possible to produce multilayer plastic articles by injection molding same from a single stream of diverse thermoplastic resinous materials wherein each article displays in its construction the character of a coextruded sheet of such materials with respect to significant portions of the article.
Specifically, it has not been possible to injection mold such a stream into an article which possesses a substantial uniformity of composition adjacent a critical surface of the article. Nor has it been possible to injection mold an article wherein overlapping layers of diverse thermoplastic resinous materials incorporated in the article have been effectively controlled in terms of their orientation and relationship with a critical surface of the article, have retained a selected layer order or have been made substantially continuous adjacent a critical surface of the article.
Parenthetically, a "critical surface" refers to a surface of the article which will be subject to various stresses or wherein the properties which are sought to be enhanced by a lamellar construction will be desired. Where the object of employing the methods and apparatus of the present invention, whether with regard to multilayer articles of a particular shape or form or with regard to the particular process of injection molding, is to enhance barrier properties with respect to a particular gas or solvent, for example, a critical surface would correspond to any surface which is expected to contact that gas or solvent and to form a barrier to permeation of the gas or solvent therethrough.
It has further not been previously possible to injection mold articles wherein one or more of the foregoing qualities are found and further, the layers are of such a thickness that a thousand layers may span but 15 microns. It has also not been considered that these qualities could fairly be achieved in multicavity injection molding using but a single stream of diverse materials or indeed, that multilayer articles showing substantially co-continuous lamellae in any selected cross-section of the articles could be produced by such multicavity injection molding, especially where one of the diverse materials is present in comparatively minor amounts.
Because of the inability to produce multilayer plastic articles having one or more of the above characteristics by a simple injection molding process, a great deal of effort and expense has been devoted to developing blends of diverse thermoplastic resinous materials which would approximate the benefits to be derived from a lamellar morphology, without the necessity of synthesizing an entirely new material or using a prohibitively expensive existing material.
Such blends are useful with standard injection molding apparatus, but possess a number of disadvantages. For example, when the blend is comprised of two or more resins which are at least partially incompatible, the morphology of a resin which is present as a minor component in the blend, as may likely be true where the particular resin is expensive, is thermodynamically driven toward the fibrillar or spherical regimes.
Where the property sought to be produced by the presence of the minor component is an improved barrier to gas or solvent permeation through the wall of an article comprised of the blend, this fibrillar or spherical morphology does not permit an effective barrier to be formed unless a substantial amount of barrier resin is added. Where the barrier resin is among the more expensive components of the blend, the addition of an amount of barrier resin sufficient to create an effective barrier and to approach a lamellar morphology in the blend can defeat the very purpose of employing a blend of otherwise relatively inexpensive components, namely, the avoidance of the expense of developing a newly synthesized material or using a more expensive existing material.
Moreover, the adhesives or compatibilizing materials which must be added generally to achieve a cohesive, unitary article comprised of such incompatible resins must find their way to an interface between the incompatible resins through a complicated flow field. Some portion of the adhesive used will not find an interface and will not accomplish its purpose, resulting in a deficiency in some properties of the article and needless materials expense.
A further disadvantage occurs when the blend of diverse thermoplastic materials is comprised of two optically dissimilar resins, i.e., the resins do not have closely related refractive indices, and a substantially transparent article is desired. When the resins have a refractive index difference greater than about 0.002, the blend will have diminished clarity due to light scattering at the interface between the resins. Light scattering is influenced by the shape of the interface between the resins, with spherical interfaces scattering more light than planar interfaces. Because blends of diverse thermoplastic resinous materials are typically prepared and fabricated by conventional blending techniques, which results in a blend morphology of a spherical or columnar type, the blends are prone to much light scattering unless the refractive indices of the resins are very closely matched.
Efforts have been made to produce multilayer articles by a co-injection molding process, as shown for example in U.S. Pat. No. 4,525,134 to McHenry et al.
Because of the complexities involved in such co-injection processes for making multilayer articles, however, and in view of the limitations and disadvantages attendant to such complexities in terms of process control, ease of operation, adaptability to various polymer systems and applications such as multicavity injection molding with cavities having varying or anything but the simplest geometries, etc., some efforts at injection molding a single stream of diverse materials have also been made and are evidenced in the art.
U.S. Pat. No. 3,716,612 to Schrenk et al., for example, discloses a method and apparatus for making a plastic article comprised of an apparent plurality of thin and distinct layers of diverse synthetic resinous materials.
The apparatus and method of the referenced Schrenk et al. patent, it has been discovered, initially create a thickness gradient in layers of the various polymeric materials to be formed into an article, and a gradient is perpetuated within the walls of the multilayer plastic article formed by such apparatus and method. Layers which are formed near a rotating wall of the annular configuration tend to become vanishingly thin and discontinuous due to the shearing movement of the wall, while other layers are comparatively thick, so that a not insubstantial portion of the layers in a cross-section of a wall of a multilayer article can become discontinuous before a desired attenuation in the thickness of other layers and parts of layers is achieved. Some corresponding loss of properties may be expected at the layers located near a rotating wall.
European Patent Application 0278403 to Mitsubishi Gas Chemical Company, Inc., represents still another effort at achieving the benefits of a lamellar construction by an injection molding process. The Mitsubishi patent application describes two methods of producing a parison and blow molded container containing a barrier resin. The first method involves the random generation of a multilayer stream of two polymers such as polyethylene terephthalate (PET) and "MX nylon" (a meta-xylene group-containing polyamide) by passing the polymers through a static mixing device. The second method achieves similar results in terms of the desired properties of the articles by merely a
- 6 coarse mixing of the same polymers. The main benefit realized by these methods is a reduction in haze compared to a uniformly mixed blend of the polymers, while maintaining or realizing a modest improvement in barrier properties and adhesion.
U.S. Pat. No. 4,410,482 to Subramanian discloses a process for manufacturing laminar articles purporting to have improved barrier properties apart from an injection molding context. Particles of a limited range of sizes of a polyolefin and a condensation polymer are gently mixed under controlled heating conditions, with a compatibilizing resin being either mixed in as individual particles or coated onto particles of the polyolefin or condensation polymer prior to mixing. The melt is then stretched as by extrusion blow molding to form a multitude of thin, substantially two dimensional, parallel and overlapping layers and is cooled.
With regard to other processes for making multilayer plastic articles, U.S. Pat. No. 4,416,942 to Diluccio discloses the coextrusion blow molding of a laminate, the first layer of which is to consist of a lamellar, heterogeneous blend of the type disclosed in U.S. Pat. No. 4,410,482 and described above, and the second layer of which is a polyolefin which comprises 10 to 90 percent of the thickness of the laminate.
It would be desirable to develop methods and apparatus for producing multilayer plastic articles wherein the articles demonstrate a substantial uniformity of composition adjacent a critical surface thereof. It would further be desirable to develop methods and apparatus for producing multilayer plastic articles wherein the layers are discrete and substantially continuous and have a selected layer order which is preserved along with one or more of the other characteristics just mentioned after the article is formed. It would additionally be desirable to develop methods and apparatus for fabricating optically dissimilar polymers into substantially transparent structures which possess a controlled lamellar morphology. It would be particularly desirable to develop such methods and apparatus which further utilize injection molding techniques and apparatus.
It is an object, then, of the present invention to provide a method and apparatus for making multilayer plastic articles utilizing injection molding techniques and apparatus.
It is also an object of the present invention to provide a method and apparatus which will produce multilayer articles wherein the layers exhibit one or more of the qualities of: a substantial uniformity of composition adjacent a critical surface of the article; the retention of discrete and substantially continuous layers adjacent a critical surface; and, the retention of a selected layer order, after injection molding, injection blow molding, compression molding, transfer molding, displacement blow molding or extrusion blow molding.
It is another object of the present invention to provide a method and apparatus for fabricating optically dissimilar polymers into substantially transparent structures.
It is a further object of the present invention to provide a method and apparatus for producing multilayer plastic articles which display one or more of the qualities just mentioned, preferably by using injection molding and injection molding apparatus, without the need for rigid, complex and expensive process controls.
Still other objects and advantages will become apparent upon reading the description which follows in conjunction with the accompanying drawings.